1. Field of the Invention:
This invention relates to a device for agitating unsolidified molten metals in continuous casting apparatus and more particularly to a device for agitating an unsolidified molten steel in continuous casting apparatus.
2. Description of the Prior Art
A segregation zone rich in carbon, sulfur and phosphorus is likely to be generated in the center portion of a slab made by continuous casting. Such a segregation zone presents a different macroscopic structure from that of a normal zone. In some cases, there is a defect that the product made of such slab has very poor mechanical properties and a low commodity value depending on its uses.
It is known that the above mentioned center segregation can be reduced by producing many equiaxed crystals in the center portion of the slab. For example, it has been suggested to agitate the unsolidified molten metal within the slab in the course of the solidification thereof.
Among the conventional methods of agitating unsolidified molten metals, there is a method A wherein a continuously cast solidifying slab is agitated by making a rotating magnetic field or a shifting magnetic field act on it and giving a thrust to the unsolidified part of the slab in the same direction as the shifting direction of the magnetic field; and a method B wherein the cast slab is agitated by making a stationary magnetic field act on the unsolidified part within the slab, making a direct current flow to the unsolidified molten metal and giving a thrust to the unsolidified molten metal by the mutual action of this current and magnetic field.
According to the method A, an agitating device must be mounted by removing a roller of a roller apron; and the agitating device must be provided with a special rigid supporting means so as to prevent the slab from bulging due to the static pressure which is proportionally larger towards the lower part of the slab, the provision of the supporting means making the whole structure complicated. Therefore, in the method A, it is impossible to mount several agitating means by removing several rollers of the roller apron as only one agitating means can be mounted. It is further impossible to mount the agitating means at the lower part of the slab.
The method A produces a non-uniform white band so that the macroscopic structure may be impaired. In order to obtain an agitating effect with one agitating means the agitation must be strongly effected so that it may result in the clear appearance of the white band. Further, since the agitation is effected in only one direction (the direction of the width of the slab), the width of the white band is liable to fluctuate.
The method B employs U-shaped permanent magnets. Such U-shaped magnets which are impossible to mount in the continuous casting machine, particularly, adjacent the slab, because for mounting the U-shaped magnets it is necessary to remove the guide rollers so that the structure may become so complicated as that mentioned in connection with the method A, and also the same defects are present as in the method A. According to the method B the agitated flow is defined by a large loop, since the N poles and S poles of the magnets are respectively on the same side of the width direction of the slab so that a non-uniform white band is clearly present.
However, according to the present invention the N poles and S poles of the magnets are arranged alternately to be opposed to each other and therefore the agitated flow describes small loops so that a uniform white band is formed.
However, in either method, a magnetic field generating device for obtaining the magnetic field is required. For the method A, there is adopted a method wherein, as shown, for example, in Japanese Published Patent application No. 33025/1972, many electromagnetic coils are paralelly mounted and opposed to each other on one or both surfaces of a slab, and alternating currents of different phase are made to flow to the respective coils. Further, for the method B, there is used a method wherein, as shown in British Patent No. 872,591, an electromagnetic is provided directly adjacent to the surface of a slab.
In order to obtain a magnetic field sufficient to agitate an unsolidified molten metal in such a magnetic field generating device, it is necessary to bring the coil as near to the slab as possible.
However, in the general continuous casting machine, many rollers are provided to rotate in contact with a slab so as to support and guide it. As seen in the above described publication or patent, unless some of these rollers are removed, the magnetic field generating device will not be able to be mounted near the slab. However, in case some of the rollers are removed, as they are, the solidified shell of the slab in such regions will be pushed and expanded outwardly by the static pressure of the molten metal. In order to prevent it, a slab supporting device of a special structure is required and the continuous casting machine becomes very complicated.
Further, generally there is a defect that, if the inner molten steel is strongly agitated in the course of the solidification, the part solidified during the agitation becomes a negative composition segregation zone called a white band. However, such a white band can be dissolved without impairing the effect of reducing the center segregation by agitating the molten metal by reducing and dividing the strength of the agitation into several steps. However, in the conventional agitating method, as described above, the agitating device is so complicated to be difficult to mount in several steps, and therefore the production of the white band can not be prevented.